1. Technical Field
Example embodiments of the present invention relate in general to a method of transmitting a dedicated reference signal (DRS), a method of receiving a DRS, and a feedback method of a terminal, and more specifically to a DRS transmitting method of a base station and a DRS receiving method of a terminal when single user multiple-input multiple-output (SU-MIMO) transmission or multi-user MIMO (MU-MIMO) transmission is performed for single point transmission or coordinated multi-point transmission (CoMP).
2. Related Art
Wireless communication systems employ a MIMO scheme to increase the frequency efficiency of wireless resources. For effective MIMO, a reference signal (RS) needs to be appropriately transmitted and received. The RS is needed to demodulate a transmitted signal and also to extract MIMO channel state information (CSI). The CSI is extracted by a receiver and transferred to a transmitter, and the transmitter generates an appropriate transmission signal using the CSI and transmits the transmission signal to another receiver.
Also, in general MIMO transmission and reception, as many orthogonal RSs as the number of transmitting layers are needed. In conventional third generation partnership project (3GPP) long term evolution (LTE) release (Rel)-8, an RS referred to as a cell-specific RS (CRS) is used in common for CSI extraction and demodulation.
In other words, since 3GPP LTE Rel-8 supports up to four antennas in a downlink, RSs for the four antennas are used to demodulate received data and also to measure and feed back CSI to a transmitter. As mentioned above, these RSs are CRSs and used for all users in a cell. Meanwhile, in a 3GPP LTE-advanced system supporting up to eight antennas in a downlink, the number of required RSs increases with an increase in the number of multiple antennas. Thus, RS overhead also increases, which is a factor deteriorating system performance.
For this reason, in LTE-advanced systems, it has been determined to prevent the increase of RS overhead and the deterioration of system performance by separately using an RS for measuring CSI and an RS for data demodulation instead of using a conventional Rel-8 CRS structure, and standardization is under way.
A demodulation RS (DM-RS) is used to demodulate a physical dedicated shared channel (PDSCH) over which data is transmitted, and a CSI-RS is used to generate the CSI of a downlink wireless channel.
In particular, a DM-RS is an RS dedicated to a specific terminal, and present in a resource block allocated to the UE. The DM-RS undergoes the same precoding as data transmitted over the corresponding PDSCH and is transmitted.
When a cell has a plurality of transmitting antennas, the following precoding MIMO scheme can be used. When the number of transmitting antennas of the cell is NT, a precoding vector has a length of NT. One precoding vector is used to transmit one symbol. In other words, when a base station wants to transmit M data symbols using one resource element (RE), M precoding vectors are used.
When the symbols transmitted by the base station are indicated by Si (i=1, . . . , and M<=NT), signals Qi (i=1, . . . , and NT) finally transmitted by an i-th transmitting antenna are expressed by Equation 1 below.
                                                                                          Q                                                                                                    →                                                  ⁢                                                                              =                              [                                                                                                    Q                        1                                                                                                                                                Q                        2                                                                                                                        ⋮                                                                                                                          Q                        N                                                                                            ]                                                                                        =                                                [                                                                                                              P                          11                                                                                            ⋯                                                                                              P                                                      1                            ⁢                                                                                                                  ⁢                            M                                                                                                                                                                                        P                          21                                                                                            ⋯                                                                                              P                                                      2                            ⁢                                                                                                                  ⁢                            M                                                                                                                                                              ⋮                                                                                                                                                                                          ⋮                                                                                                                                      P                                                                                    N                              T                                                        ⁢                                                                                                                  ⁢                            1                                                                                                                      ⋯                                                                                              P                                                                                    N                              T                                                        ⁢                                                                                                                  ⁢                            M                                                                                                                                ]                                ⁡                                  [                                                                                                              S                          1                                                                                                                                                              S                          2                                                                                                                                    ⋮                                                                                                                                      S                          M                                                                                                      ]                                                                                                        =                                                [                                                                                                                                                                        P                              1                                                                                                                                                                                  →                                                                                ⁢                                                                                                                                                                            ⋯                                                                                                                                                        P                              2                                                                                                                                                                                  →                                                                                ⁢                                                                                                                                                                                      ]                                ⁡                                  [                                                                                                              S                          1                                                                                                                                                              S                          2                                                                                                                                    ⋮                                                                                                                                      S                          M                                                                                                      ]                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Here,
            P      i                                  →        ⁢          =            [                                                  P                              1                ⁢                                                                  ⁢                i                                                                                        P                              2                ⁢                                                                  ⁢                i                                                                          ⋮                                                              P                                                N                  T                                ⁢                                                                  ⁢                i                                                        ]        ⁢          (                        i          ⁢                                          =                                          ⁢          1                ,        2        ,        …        ⁢                                  ,                  and          ⁢                                          ⁢          M                    )      is a precoding vector corresponding to an i-th transmission symbol Si (i=1, . . . , and M).
A DM-RS undergoes the same precoding as data and is transmitted. When reference symbols are indicated by Xl (l=1, . . . , and M), an RS Qi(l) finally transmitted for each reference symbol Xl by an i-th transmitting antenna is expressed by Equation 2 below.
                                                        Q                              (                l                )                                                                                                →                                ⁢                                                =                              [                                                                                Q                    1                                          (                      l                      )                                                                                                                                        Q                    2                                          (                      l                      )                                                                                                                    ⋮                                                                                                  Q                                          N                      T                                                              (                      l                      )                                                                                            ]                    =                                                    P                l                                                                                              →                                      ⁢                                                  ⁢                                          X                l                            ⁢                                                          (                                                1                  ⁢                                                                          =                                                                          ⁢                  1                                ,                …                ⁢                                                                  ,                                  and                  ⁢                                                                          ⁢                  M                                            )                                                          [                  Equation          ⁢                                          ⁢          2                ]            
When the number of symbols simultaneously transmitted by a base station is M, M precoding vectors are used, M DM-RSs are needed to estimate the channel of a terminal, and each RS undergoes the same precoding as the corresponding transmission symbol.
When a base station simultaneously transmits M data symbols using one RE, data symbols transmitted to a specific terminal during one transmission time interval (TTI) can be grouped into M layers. In other words, an i-th layer denotes a symbol group consisting of i-th transmission symbols of respective REs.
To estimate a channel using the DM-RS or CSI-RS of a terminal, the terminal needs to know the positions of REs through which the RSs are transmitted and an RS sequence used as an RS. At this time, different cells may configure DM-RSs and CSI-RSs to occupy different positions (i.e., show different patterns) in a time-frequency resource region and be orthogonal to each other.
A DRS of antenna port 5 defined in a current 3GPP LTE Rel-8 system is configured in the same format as a DM-RS of the aforementioned LTE-advanced system.
FIG. 1 shows disposition of time-frequency resources for illustrating the pattern of an antenna port 5-DRS of conventional LTE Rel-8.
A terminal determines the position of an RE through which an RS of antenna port 5 is transmitted according to the physical layer cell identity (PCI) of a serving cell, and determines an RS sequence on the basis of the PCI and the radio network temporary identifier (RNTI) of the terminal. Thus, the terminal can specify the position of a DRS of antenna port 5 and a DRS sequence using the PCI of the serving cell and the RNTI and receive the DRS of antenna port 5 and the DRS sequence.
However, CoMP technology is expected to be introduced to LTE-advanced or the following next-generation mobile communication, and each terminal needs to receive a DM-RS transmitted from another cell as well as a DM-RS transmitted from the serving cell of the terminal itself and perform channel estimation.
Also, when MU-MIMO is applied to single cell transmission (single point transmission), a terminal needs to receive the DM-RSs of other terminals receiving data from the same cell together with the terminal itself, estimate interference caused by the other terminals, and prevent or remove an interference signal to improve performance. This is because LTE-advanced supports dual or more layer beamforming and RSs for two or more users can be mapped to the same RE, unlike the antenna port 5 RS of conventional Rel-8 shown in FIG. 1 in which only beamforming for a single user is considered and RSs for two or more users are not mapped to the same RE.